1. Field of the Invention
The present invention relates to a displacement detection device for detecting physical displacement of an object or a liquid level utilizing the magnetostriction phenomenon and to a magnetostrictive wire therefor.
2. Description of the Related Art
U.S. Pat. No. 3,173,131 discloses a magnetostrictive apparatus for displacement detection comprising a magnetostrictive wire, a permanent magnet movable along the wire, an oscillator means for applying a pulse current to the wire, and a receiver means disposed at a selected portion of the wire for receiving an ultrasonic wave or a magnetostriction signal generated in the wire in a portion close to the permanent magnet.
A pulse current applied by the oscillator means to the wire generates a circumferential magnetic field around the wire over the entire length thereof and the permanent magnet also generates an axial magnetic field along the wire only in the portion close thereto, so that the wire in that portion is twisted or distorted through the Wiedemann effect and the sudden occurrence of the distortion forms a twisting vibration or an ultrasonic wave which propagates toward both ends of the wire. Detection of this ultrasonic wave by a receiver means allows a distance L along the wire from the receiver means to the permanent magnet to be given as a function of time t as expressed by: EQU L=V.multidot.t
where V represents a propagation speed of the ultrasonic wave and is given by the shear modulus G of the magnetostrictive wire and the density .rho. of the wire as expressed by: EQU V=(G/.rho.).sup.1/2.
When a wire is made of Ni, a material commonly used for the wire, the shear modulus G and the density .rho. vary with the temperature change, which causes the propagation speed to vary and an error in the distance measurement to occur.
Japanese Unexamined Patent Publication No. 2-183117 discloses a magnetostrictive wire made of an Elinvar alloy such as "NiSpanC" (trade name), which is a constant-modulus alloy having a modulus which does not vary with temperature. The temperature coefficient of the ultrasonic wave propagation speed of the Elinvar alloy can be reduced to 20 ppm/.degree.C. or less by heat treatment or other processing conditions. In contrast, the measurement error due to temperature change in the detection circuitry generally ranges from 200 to 500 ppm/.degree.C. Therefore, the variation of the ultrasonic wave propagation speed of the Elinvar alloy can practically be ignored. Thus, the Elinvar alloy is advantageously used as a material of magnetostrictive wire because of its small temperature coefficient of the resonance frequency and a stable magnetostriction transfer speed or twisting vibration speed which does not vary with temperature.
On the other hand, the Elinvar alloy has a magnetostrictive coefficient (.lambda.) as small as about 5.times.10.sup.-6, and therefore, the displacement detection requires amplification at a high speed of response. Moreover, the magnetostrictive coefficient is slightly reduced at temperatures above 100.degree. C., which also causes an error in the displacement detection.